1. Plant hormones auxins gibberellins abscisic acid ethylene cytokinins
brassicosteroids

2. Response to light: Phototropism
Growth towards light
Hormone: Auxin
asymmetrical distribution of auxin
cells on darker side elongate faster than cells on brighter side

3. Apical dominance Controls cell division & differentiation
axillary buds do no grow while apical bud exerts control
shoot
root

4. Gibberellins Family of hormones Effects
over 100 different gibberellins identified
Effects
stem elongation
fruit growth
seed germination
plump grapes in grocery stores have been treated with gibberellin hormones while on the vine

5. Abscisic acid (ABA) Effects slows growth seed dormancy
high concentrations of Abscisic acid
germination only after ABA is inactivated down or leeched out
survival value: seed will germinate only under optimal conditions
light, temperature, moisture
drought tolerance
rapid stomate closing

6. Ethylene Ethylene is a hormone gas released by plant cells
Multiple effects
response to mechanical stress
triple response
slow stem elongation
thickening of stem
curvature to stem growth
leaf drop (like in Fall)
apoptosis
fruit ripening

7. Apoptosis & Leaf drop Ethylene & auxin
What is the evolutionary advantage of loss of leaves in autumn?
Ethylene & auxin
many events in plants involve apoptosis (pre-programmed cell death)
death of annual plant after flowering
differentiation of xylem vessels
loss of cytosol
shedding of autumn leaves
The loss of leaves each autumn is an adaptation that keeps deciduous trees from desiccating during winter when the roots cannot absorb water from the frozen ground. Before leaves abscise, many essential elements are salvaged from the dying leaves and are stored in stem parenchyma cells. These nutrients are recycled back to developing leaves the following spring. Fall color is a combination of new red pigments made during autumn and yellow and orange carotenoids that were already present in the leaf but are rendered visible by the breakdown of the dark green chlorophyll in autumn.
Photo: Abscission of a maple leaf.
Abscission is controlled by a change in the balance of ethylene and auxin. The abscission layer can be seen here as a vertical band at the base of the petiole. After the leaf falls, a protective layer of cork becomes the leaf scar that helps prevent pathogens from invading the plant (LM).

8. Fruit ripening Adaptation Ethylene
hard, tart fruit protects developing seed from herbivores
ripe, sweet, soft fruit attracts animals to disperse seed
Ethylene
triggers ripening process
breakdown of cell wall
softening
conversion of starch to sugar
sweetening
positive feedback system
ethylene triggers ripening
ripening stimulates more ethylene production

9. Applications Truth in folk wisdom!
one bad apple spoils the whole bunch
ripening apple releases ethylene to speed ripening of fruit nearby
Ripen green bananas by bagging them with an apple
Climate control storage of apples
high CO2 storage = reduces ethylene production

10. Flowering Response Triggered by photoperiod
relative lengths of day & night
night length—“critical period”— is trigger
Plant is sensitive to red light exposure
What is the evolutionary advantage of photoperiodism?
Synchronizes plant responses to season
Short-day plants
Long-day plants

11. Circadian rhythms Internal (endogenous) 24-hour cycles 4 O’clock
Noon
Midnight
Morning glory

12. Response to gravity
How does a sprouting shoot “know” to grow towards the surface from underground?
environmental cues?
roots = positive gravitropism
shoots = negative gravitropism
settling of statoliths (dense starch grains) may detect gravity

13. Response to touch Thigmotropism
Mimosa (Sensitive plant) closes leaves in response to touch
Caused by changes in osmotic pressure =
rapid loss of K+ =
rapid loss of H2O =
loss of turgor in cells

14. Plant defenses
Defenses against herbivores

15. Plant defenses coevolution Defenses against herbivores
Parasitoid wasp larvae emerging from a caterpillar